The small feature size or line width, which is characteristic of integrated circuits has required the development of precise method and apparatus for aligning two objects with each other, for example, a mask and wafer. This development has led to the practice of a variety of alignment techniques, the variety of techniques is indicated by the fact that the various techniques employ optical energy or light, electron beams, and even X-rays for the illumination function. Typical of the prior art techniques are Smith et al, U.S. Pat. No. 3,742,229; Davis et al, U.S. Pat. No. 3,901,814; Michail et al, U.S. Pat. No. 3,900,736; Suzki, U.S. Pat. No. 4,167,677; O'Keeffe et al, U.S. Pat. No. 3,840,749; Johannsmeier et al, U.S. Pat. No. 4,070,117 and U.S. Pat. No. 3,683,195; and Nakazawa et al, U.S. Pat. No. 4,103,998.
In order to realize the full potential of several very high resolution fabrication techniques it is essential to provide a method and apparatus for registering successive masks with a wafer with sufficient accuracy. For example, for sub-micron lithography (minimum feature size between 0.2 and 0.5 microns) it has been estimated that the required alignment accuracy is on the order of .+-.0.5 microns. In attempts to push alignment accuracy to this region, prior art workers have gone to the use of collective interference effects which take place between alignment marks exhibiting a grating structure. For example, one group of alignment techniques employ the Moire effect; in this regard see Reekstin et al, U.S. Pat. No. 4,193,687, "Uber Die Justierung Ebener Strukturen Mittels Moire" by Schwieder et al in Optica, Volume 23, pages 4961 (1976) and "Photolithographic Mask Alignment Using Moire Techniques" by King et al in Applied Optics, Volume 11, No. 11, pages 2455-2459 (November, 1972). The problem with Moire techniques in general is that while they appear to be capable of extending the accuracy of conventional optical methods, the difficulty lies in achieving sufficient contrast.
Also reported is an alignment method using identical gratings on mask and wafer, for example see Smith et al, U.S. Pat. No. 4,200,395.
Finally, very recently Fay et al reported in "Optical Alignment System for Sub-Micron X-ray Lithography", J. Vac. Sci. Technol., Volume 16, No. 6, pages 1954-1958 (November-December, 1979) an optical alignment method which embraces the concept of Fresnel zone lenses, optical scan and diffraction grating. The report claims extreme accuracy, better than .+-.0.05 microns, continuous alignment, gap monitoring and high signal to noise ratio. Another alignment technique employing Fresnel zone plates for focusing a light beam is described in Feldman et al, U.S. Pat. No. 4,037,969.
As described by Fay et al, their alignment technique proposes a Fresnel zone plate on one of the two elements to be aligned, and a reflective stripe, on the other element. For example, Fay et al propose the zone plate on the mask, and the stripe on the wafer. The zone plate has the property of focusing incident light, so that, for example, by adjusting the proximity gap (distance between wafer and mask) to be equal to the focal length of the zone plate light incident on the zone plate is focused on the wafer. The reflective stripe on the wafer has a reflection coefficient which is larger than its surroundings. In order to obtain good contrast; to reduce background and other unwanted interference effects, Fay et al propose making the reflecting stripe actually a grating so as to limit or eliminate the zero order reflection from the zone plate. Because the reflective stripe is essentially a grating the light reflected from the wafer mark is diffracted into many diffraction orders. A photodetector, which is used to detect reflected light, is located so as to collect the first or any higher diffraction order (practically, the first order is the best) and the detected signal will be completely free of zero order reflection. The first order diffraction signal then will produce a convolution signal with very low background when mask and wafer and relatively displaced. Fay et al employ a scanning mirror in the optical path between the source of light and the mask-wafer, so that by rotating the mirror the angle of incidence of radiation on the mask is varied which results in displacing the incident radiation on the wafer. This enables an error signal to be derived without required relative motion between mask and wafer, which error signal can be used to drive the relative displacement error between mask and wafer to zero.
In the experiment reported by Fay et al the scanning motion of the mirror resulted in motion at the wafer of about .+-.1 micron. Fay et al also suggests that the wafer grating (typically 1.5 microns in width) be alternated with a wider grating (10 microns in width) with different grating periods such that two alignment signals are returned in slightly different directions. While Fay et al suggests that the signal from the narrow lines could be used for fine alignment and the larger lines could be used for pre-alignment, it is not at all apparent what the form of the signal from the larger lines would be, or how that could be used in pre-alignment.
Thus, a difficulty with the Fay et al alignment technique is its limited range. While manual coarse alignment is certainly possible, we do not believe that manual alignment is capable of efficiently reducing the mask-wafer misalignment to the .+-.1 micron range of the Fay et al alignment technique.
It is therefore one object of the present invention to provide an improved alignment technique which is not limited in range as apparently Fay et al's technique is. It is another object of the present invention to provide an alignment apparatus, generally of the type described by Fay et al but in which the wafer mark has characteristics not described by Fay et al, so as to enable a substantially unlimited increase in automatic alignment range for the apparatus. It is another object of the present invention to provide an alignment system of the foregoing type which is capable of generating an error signal having any desired relation to misalignment. For example, in one embodiment of our invention the wafer mark is arranged such as to provide a linear relation between misalignment and resulting error signal; however, by varying the characteristics of the alignment mark on the wafer, it is also within the scope of our invention to provide a non-linear relation between error signal and misalignment, for example, an error signal with derivatives increasing with misalignment. While embodiments of our invention employ an alignment mark which is arranged to provide an error signal which is, in an analog sense, a measure of the misalignment, it is also within the scope of our invention to provide an alignment mark which is arranged to provide an error signal which is digitally encoded to represent misalignment. The alignment mark providing a digitally encoded error signal can, if desired, be arranged to provide, in addition to the digitally encoded error signal, an analog error signal which is a measure of the misalignment.
It is a further object of the invention to provide mask-wafer gratings to ensure unambiguous alignment as compared to ambiguous alignment which can result if a periodic Fresnel zone plate is employed. It is yet another object of the invention to maximize the useable diffracted light. Other objects of the invention will become apparent as this description proceeds.